The invention relates to a light modulation device for a display device, in particular a holographic display device, for the representation of two-dimensional and/or three-dimensional reconstructed scenes by means of which, after the incorporation of the light modulation device into a display device, the risks resulting from the exposure of persons observing the display device to the light radiation emitting from a light source device are avoided. The invention also relates to a display device comprising such a light modulation device.
Light modulation devices are used mainly for modulating incident light in display devices, also referred to as displays. Such light modulation devices comprise modulation elements (so-called pixels), which modulate the incident light according to their control. These modulation elements are formed by a controllable layer in the light modulation device, which is mostly based on the use of liquid crystals (LC). In display devices, it is, of course, also possible to use light modulation devices that are not based on liquid crystals but are based, for example, on electrowetting or the use of micro-electro-mechanical systems (MEMS).
Light modulation devices, in particular for the holographic display of two-dimensional (2D) and/or three-dimensional (3D) reconstructed scenes, are mostly illuminated using a light source device that emits sufficiently coherent light. Such usable light sources are, for example, lasers or even LEDs (light emitting diodes). However, the use of lasers requires increased safety measures due to the possible serious danger of laser light for person exposed to the region of the laser beam. For example, burns at specific sites on the body of persons or damage to the eyes may occur, or even the loss of sight cannot be ruled out if laser light enters the eyes of persons. For this reason, when lasers of certain power are used, adequate safeguard measures need to be taken, such as complying with the safeguards set by the individual countries.
Regarding the safety of two-dimensional (2D) display devices using lasers as light sources, such as projectors or laser TV; these display devices usually comprise a scatter foil, which improves the homogeneity, to ensure the laser safety. As a result, the surface of the display device or the screen on which an image is projected, acts as a spatially incoherent illuminating surface that is imaged not on a single point but on the area of the eyes of persons.
A further option for obtaining a high laser safety is disclosed in DE 196 40 404 A1. The device for displaying images on a projection screen comprises an image-generating device and a laser as a light source, where the image-generating device can be operated in two operating modes, of which the first is the normal operating mode and the second is an operating mode where the laser radiation is harmless for persons. In order to avoid eye damage, the image-generating device is switched into the second operating mode when a sensor detects a person who is directly in the beam path of the laser.
When using holographic display devices, however, no such scatter foils as mentioned above can be used because coherent light is needed for a holographic reconstruction. The coherence of light is reduced or even destroyed by such scatter foils.
Usually a hologram is encoded into the light modulation device, where the hologram causes a scattering of light and distributes the light within the room during operation of the light modulation device, in other words, during the controlling of the light modulation device. However, it may become critical and dangerous to persons if the light modulation device or the control of the light modulation device fails, causing the entire light to enter or be brought into the focus of a diffraction order used, such as the zeroth diffraction order.
This will be explained briefly by the following example: A state-of-the-art holographic display device comprises a light source device, usually a laser light source, a spatial light modulator, beam-deflecting means, and beam-shaping means, for example lenses. The design and functioning of such a display device are described, for example, in DE 103 53 439 B4.
The display device and the figures of DE 103 53 439 B4 shall illustrate a state-of-the-art holographic display device and the present invention, where FIGS. 1 and 2 of the present invention shall illustrate the section of the display device of DE 103 53 439 B4, which is important for the invention only in principle. FIG. 1 shows a side view of a holographic display device 1. For reasons of clarity, the individual elements or components of the display device 1 are not illustrated here. The display device 1 comprises, for example, a light source device, a spatial light modulator as well as beam-deflecting means and/or beam-shaping means. The light modulated in the display device 1 exits said display device in a bundle of light beams 2 and illuminates a virtual viewing window 3. A three-dimensional (3D) scene 4 is visible through this virtual viewing window 3 as a holographic reconstruction when one eye of an observer coincides with the position of the virtual viewing window 3. Here, the virtual viewing window 3 is positioned in the Fourier plane of the spatial light modulator. A field lens focuses the light emanating from the spatial light modulator, where the focus of the field lens is in the plane of the viewing window 3. FIG. 2 illustrates how, in the direction of light propagation, the bundle of light beams 2 form a focus F behind the holographic display device 1 if the spatial light modulator allows the light to be passed in an unmodulated state. This may be the case, for example, in the event of a malfunction of the holographic display device 1, where control means for controlling the spatial light modulator and the beam-deflecting means do not function properly.
Focusing the bundle of light beams 2 on a focus F affects the laser safety of the display device 1 and can compromise the health of the observers' eyes. On the one hand, all the light in the bundle of light beam 2 is focused on the focus F and can, therefore, hit completely an observer's eye or eye pupil. According to FIG. 1, the light is usually distributed over the area of the viewing window 3, thus only partially hitting an observer's eye or eye pupil. On the other hand, concerning the laser safety of a holographic display device, the angular extension of the light source used is to be considered. According to the Accident Prevention Regulation BGV B2 “Laser Radiation” (German laser safety regulation BGV B2 that is compatible with international regulation IEC 60825), this angular extension is included both in a correction factor C6, which is required by the Accident Prevention Regulation BGV B2 “Laser Radiation”, and in the calculation of the maximum permissible radiation. A light source having a small angular extension is imaged on a small area of the retina of an eye and is hence more dangerous than a light source having a larger angular extension of the same power. A light source having a small angular extension is the case when there are no scattering optical elements in the beam path of the laser light. In this case, there is a diffraction-limited point image of the light source on the retina of the eye. On the other hand, there is, among others, a light source having a large angular extension when, according to FIG. 1, a three-dimensional scene 4 having a spatial extension is reconstructed holographically and imaged on the retina of the eye.
Hence, a malfunction of the holographic display device can lead particularly to eye damage of an observer caused by laser radiation.